This application claims priority benefits of prior filed co-pending British patent application No. 00 02277.2, filed Feb. 1, 2000, entitled, INTEGRATED OPTICAL COMPONENTS, and is incorporated herein by reference thereto.
The present invention relates to an optical component and in particular to a component having an optical waveguide formed in a substrate.
The manufacture of integrated optical components intended for use in the telecommunications industry for the transmission of data with optical signals is well known in the art. A typical manufacturing technique for such a component is the employment of a lithium niobate substrate cut from a wafer of this material and thereafter forming an optical waveguide or waveguides including branched waveguides or Mach-Zehnder configurations of the required configuration in the surface of the substrate. The optical waveguide of the required configuration may be formed in the substrate such as by a selective titanium diffusion process. The substrate is then processed in a manner to provide it with the required operating characteristics. For example, an electrode structure may be formed on the surface of the substrate, spatially with the longitudinal extent of the formed optical waveguide so that electrical signals supplied to the electrode structure may influence the propagation and characteristics of an optical signal along the waveguide. The substrate is then mounted in a package appropriate with input and output optical and electrical connections providing for signal coupling and controlling of the operation of the component. The packaged component may, then, be deployed within an optical communication system.
Before a required electrode structure can be formed on the substrate, a buffer layer is deposited on the surface of the substrate. The buffer layer is, for example, a layer of silicon dioxide or some other dielectric material. After the formation of the buffer layer, the electrode structure is formed on the buffer layer as is known in the art. The buffer layer isolates the metal electrodes from the waveguides and also allows the realization of required microwave performance. The use of a buffer layer serves as increased spatial separation of the electrodes from the optical waveguide formed in the substrate. Consequently, the overlap between the optical field within the waveguide or waveguides, with respect to the electrical field produced between the electrodes of the electrode structure, will depend upon the thickness of the buffer layer, as well as the physical and electrical properties of the buffer layer. As a result, it is generally necessary to employ a drive voltage for the electrodes which is higher than might otherwise be required in view of the presence of the buffer layer.
It is an object of the present invention to reduce the magnitude of the drive voltage required for operation of electrodes utilized in the control of an optical device.
According to this invention, an optical component comprises a substrate in which is formed an optical waveguide and an electrode structure formed on a surface of the substrate to influence propagation of the light along the waveguide when driven with a suitable control signal, the electrode structure comprising at least one ground electrode and a signal electrode. The ground electrode is formed on the surface of the substrate and a buffer layer is formed between the surface of the substrate and the signal electrode. Advantageously, embodiments of the present invention enhance the performance of an optical system employing such an integrated optical component to allow a reduction in the drive voltage while still achieving desired performance characteristics.
It will be appreciated that in achieving the objective of this invention that, in the embodiments of the present invention, the ground electrodes are formed directly on the surface of the substrate, i.e., in particular, a buffer layer is not provided on the surface of the substrate in the region of the ground electrode, although a buffer layer is provided between the signal electrode and the substrate. The buffer layer must be sufficiently thick to reduce optical absorption losses induced by the proximity of the signal electrode to the waveguides. The lack of a buffer layer between the ground electrode and the substrate allows a reduction in the required drive voltage.
A typical integrated optical component may have more than one optical waveguide and consequently more than one electrode structure on the substrate. In such a case, it may be necessary to provide a buffer layer between more than one electrode and the substrate to achieve a desired reduction in optical losses. For example, in the case of a Mach-Zehnder modulator, there may be two branch waveguides with ground electrodes disposed either side of the two branches, and a single signal electrode disposed between the two branches. In such an arrangement, the signal electrode alone may be isolated from the substrate by means of a buffer layer, and the two ground electrodes are provided directly, without the intermediary of a dielectric buffer layer, on the upper surface of the substrate. However, in the application of a chirped modulator, a buffer layer is provided between one of the ground electrodes and a corresponding waveguide positioned under that ground electrode.
Empirical tests of various embodiments have shown that it is possible to reduce the drive voltage supplied to the signal electrode by 5% to 15%, without compromising the performance of the optical component. Having regard to the very high frequencies at which a typical integrated optical component is required to operate, a reduction in voltage of this order of magnitude is most significant, and so greatly reduces the design specification with which the drive electronics must comply.
Preferably, the substrate for realizing an embodiment of the present invention will be an x-cut lithium niobate substrate. Accordingly, the buffer layer will preferably be a layer of silicon dioxide. Such a buffer layer may be formed by depositing over the entire surface of the substrate a layer of silicon dioxide and then chemically removing that portion where the layer is not required. Other dielectric materials, as known and used in the art, may be used to form the buffer layer. Alternatively, the region where a layer is not required may be masked prior to silicon dioxide deposition in order to produce a buffer layer only in regions where subsequently formed electrodes are dialectically isolated from the waveguides.